1. Field of the Invention
The invention relates to a system based on a phosphonic acid derivative and on a metabisulphite, for stabilizing ascorbic acid, to the use of this system in compositions containing ascorbic acid, and to compositions containing it in particular in the cosmetics and/or dermatological fields.
The invention also relates to the use of the compositions obtained for the cosmetic treatment of human skin, as well as for the preparation of a cream or ointment intended for the dermatological treatment of human skin. These compositions can be applied topically, to the face, including the area around the eyes, to the body and to the scalp of human beings.
2. Discussion of the Background
It has been sought for a long time to stabilize ascorbic acid, or vitamin C, in appropriate pharmaceutical presentations, on account of its beneficial properties.
Ascorbic acid has many biological functions, such as the stimulation of collagen synthesis, the strengthening of skin tissues against external attacking factors (UV radiation, pollution), depigmentation, its anti-free-radical activity, and the compensation for vitamin E deficiency. Some of these beneficial properties have been reported in particular by England and Seifter in the article "The biochemical functions of ascorbic acid" published in Ann. Rev. Nutri., 1986, vol. 6, p. 365-406.
However, on account of its chemical structure (a-keto lactone), ascorbic acid is very sensitive to the influence of environmental parameters such as light, oxygen and water (due to its pH and due to the presence of traces of metals). This results in an unavoidable degradation over time of ascorbic acid in solution form. This degradation is reflected by a colour change and by the generation of carbon dioxide.
This problem has been approached in many ways in the prior art.
To reduce or delay the degradation of ascorbic acid in solution form, it has been recommended in document U.S. Pat. No. 5,140,043 to stabilize by introducing it into aqueous-alcohol solutions, formed of at least 80% water and having a pH of less than 3.5. On account of the strong acidity of these solutions, it is difficult to envisage them being used in the cosmetics and/or pharmaceutical field, since repeated application of these solutions can disrupt the skin's equilibrium and in particular irritate the skin or even burn it. In addition, at acidic pH, although the colour change is stabilized, the generation of carbon dioxide (CO.sub.2) is not.
The article by B. R. Hajratwala entitled "Stability of ascorbic acid" published in the Revue Sciences Pharmaceutiques on Mar. 15, 1985 is moreover known. That article teaches in particular stabilizing ascorbic acid in aqueous acidic solution by adding a surfactant which is an oxyethylenated sorbitan ester. In particular, the author reported therein that at pH 3.4 and at 25.degree. C., adding this agent decreased the rate of oxidation, and thus the rate of degradation of the ascorbic acid in solution. In addition, that document teaches the use of a chelating agent such as ethylene-diaminetetraacetic acid (EDTA), and conditioning under nitrogen in the absence of light, in order to improve the stability of ascorbic acid in aqueous solution. Such an aqueous acidic solution, applied to the skin, has the same drawbacks as those described above for the aqueous-alcoholic acidic solutions. Furthermore, the stabilization obtained is still insufficient.
Other ways of stabilizing ascorbic acid have been envisaged, in particular by coating (technique described in document FR-A-1,600,826) or by granulating ascorbic acid (technique illustrated in document JP-A-53-127,819, for the agrifood sector). However, these techniques are expensive, on the one hand, and can adversely affect the ascorbic acid, on the other hand, for example during heating, and/or can lead to uncosmetic compositions, as is the case with granules.
It is moreover known, from document FR-A-1,489,249, to use metal salts of phosphorylated ascorbic acid, in particular magnesium ascorbyl phosphate, in cosmetic compositions. The latter compound has a level of activity close to that of ascorbic acid, from which it is derived, but it has certain drawbacks which render its use on the skin unlikely. In particular, since magnesium ascorbyl phosphate is only stable at basic pH (pH 8 to pH 9), it must be incorporated into a basic composition which may be irritant to the skin (which has a pH value of about 5.5).
In addition, document EP-A-670,157 describes an emulsion containing stabilized ascorbic acid. However, this is a specific pharmaceutical form (water-in-oil emulsion) and it is desirable to be able to stabilize ascorbic acid in compositions in very varied pharmaceutical forms.
Consequently, the set of proposals which have been made hitherto has not made it possible to solve the technical problems associated with the instability of ascorbic acid, in any pharmaceutical form for the cosmetics and/or dermatological fields and at a cost which is compatible with the industrial requirements.
There is thus a need for a composition which can be used in particular in the cosmetics and/or dermatological fields, containing stabilized ascorbic acid, in a free form, i.e. which has no additional group acting in particular as stabilizer, which causes no skin irritation when it is applied, which can be in any pharmaceutical form and which is stable both as regards the colour and as regards the evolution of CO.sub.2.
The Applicant has now found a system based on a phosphonic derivative and on metabisulphite, which allows ascorbic acid to be stabilized. A considerable synergistic effect of the system according to the invention is observed.
Admittedly, it is known to use phosphonic acid derivatives in compositions containing vitamin C (see EP-A-670,157 mentioned above). It is also known to stabilize vitamin C with metabisulphite (see JP-A-6-247,855). However, the synergism of the combination claimed had never been observed.